Lithographic printing plate compositions frequently include a print-out composition comprising a leuco dye and a photooxidant (also called an accelerator or sensitizer in the literature). When the composition is exposed to activating radiation, the photooxidant converts the leuco dye to a form having a color different from that of the leuco dye. The use of the print-out composition in the printing plate is advantageous in that exposure levels given to the composition can be checked visually. If the exposures are determined to have been too low, or portions of the composition are seen to be unexposed, the plate can be re-exposed. To be effective for this purpose, the print-out composition must generate sufficient dye density upon exposure to be readily observed, without sacrificing significantly the speed of the plate. As is well known, the absorption by the photooxidant and/or the dye that prints up is selected so that it does not significantly interfere with the activation of the photopolymer.
Prior art print-out compositions have featured primarily carboxylic acid derivative photooxidants. Examples of such photooxidants, used in lithographic printing plates, are identified in U.S. Pat. No. 3,359,109, issued on Dec. 19, 1967. Many classes of useful leuco dyes are identified in the aforesaid patent. Useful photooxidants described in that patent include, N,N,O-triacylhydroxyl amines. A number of such photooxidants have been found to produce only faint print-out densities when used with, e.g., the tris(p-aminoaryl)methane leuco dyes appearing in the list of dyes in the '109 patent. Therefore, such photooxidants are of limited utility as the faint densities are difficult to observe.
Thus, prior to this invention there has been a need for a print-out composition that produces a readily detectable print-out density on exposure to activating radiation.